Google guava 集合簡介

周冠甫 2019/12/25 10:20:23

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前言

Guava是一個Goolge開源的Java通用library，包含多種被google的java專案廣泛使用的核心庫，例如：集合、快取、字串處理、I/O、算術運算、反射、多執行序等工具。本文將介紹如何導入guava lib 及 guava collection的使用。

環境

IDE: intelliJ

JDK: 1.8

Framework: Spring-boot.2.1.10.RELEASE

Build Tool: Maven 3

Server: tomcat8

導入library

Maven

<dependency>
  <groupId>com.google.guava</groupId>
  <artifactId>guava</artifactId>
  <version>28.1-jre</version>
  <!-- or, for Android: -->
  <version>28.1-android</version>
</dependency>

Gradle

dependencies {
  // Pick one:

  // 1. Use Guava in your implementation only:
  implementation("com.google.guava:guava:28.1-jre")

  // 2. Use Guava types in your public API:
  api("com.google.guava:guava:28.1-jre")

  // 3. Android - Use Guava in your implementation only:
  implementation("com.google.guava:guava:28.1-android")

  // 4. Android - Use Guava types in your public API:
  api("com.google.guava:guava:28.1-android")
}

實作範例

ImmutableList

當我們的集合會被不可信任的函數調用時，一定會擔心集合內的資料被竄改，

Guava collection 為各集合提供了三種建立不可變集合的方法，讓你不用擔心資料被更動。

1. copyOf : 從原集合複製出不可變更的集合

List<String> animalList = new ArrayList<>();
animalList.add("cat");
animalList.add("dog");
ImmutableList<String> imutableUserList = ImmutableList.copyOf(animalList);

2. of : 直接建立不可變集合

ImmutableList<String> imutableUserList = ImmutableList.of("cat", "dog");

3. build : 另外一種直接建立的方法

ImmutableList<String> imutableUserList = ImmutableList.<String>builder()
        .add("cat")
        .add("dog")
        .build();

若嘗試修改集合中的資料

ImmutableList<String> imutableUserList = ImmutableList.copyOf(animalList);
imutableUserList.add("bird");

會跳出 java.lang.UnsupportedOperationException

List

1. 過濾 : 依參數條件過濾集合

ArrayList<String> list = Lists.newArrayList("java", "ptt", "javaScript", "python", "json");

Collection<String> result = Collections2.filter(list, (e) -> e.endsWith("t"));
result.forEach(System.out::println);

2. 拆分 : 將list依照設定大小拆分

List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
// 以size：2 拆分
List<List<Integer>> listPartitionList = Lists.partition(list, 2);

System.out.println("原 List:");
System.out.println(list);

System.out.println("拆分後:");
for (List<Integer> partition: listPartitionList) {
    System.out.println(partition);
}

Set

Set<Integer> set1 = Sets.newHashSet(1, 2, 3);
Set<Integer> set2 = Sets.newHashSet(3, 4, 5);

1. 交集

Sets.SetView<Integer> intersection = Sets.intersection(set1, set2);
intersection.forEach(System.out::println);

2. 差集

Sets.SetView<Integer> difference = Sets.difference(set1, set2);
difference.forEach(System.out::println);

3. 並集

Sets.SetView<Integer> union = Sets.union(set1, set2);
union.forEach(System.out::println);

以下介紹guava加入的幾種新集合類型：

Multiset

與一般set不同，元素可重複

String str = "have have a nice nice day";
String[] ss = str.split(" ");

HashMultiset<String> multiset = HashMultiset.create();
for (String s : ss) {
    multiset.add(s);
}

Set<String> set = multiset.elementSet();

// 查看相同元素的個數
set.forEach(s -> System.out.println(s + " : " + multiset.count(s)));

BiMap

key與value可互為鍵值，但鍵與值不可重複

BiMap<String, String> biMap = HashBiMap.create();
biMap.put("cat", "fish");
biMap.put("dog", "meat");
biMap.put("rabbit", "grass");

// 可使用 inverse 將 key、value 反轉
String key = biMap.inverse().get("fish");
System.out.println("key：" + key);

Table

擁有"row"和"column"兩個鍵值的Map

Table<Object, Object, Object> table = HashBasedTable.create();
table.put("Felix", "java", 95);
table.put("Eric", "python", 90);
table.put("Beef", "Android", 89);
table.put("Owen", "PHP", 88);

Set<Table.Cell<Object, Object, Object>> cells = table.cellSet();
cells.forEach(c -> System.out.println(c.getRowKey() + "-" + c.getColumnKey() + " : " + c.getValue()));